I am using the MicroSoft BI Portal Sample Application for Windows
SharePoint Services.  My site is something like http://myserver/bip

I have created 3 document libraries, one each for 3 different groups
within the company (Marketing, Sales, and Finance).  I have also
created 3 site groups, so that each site group has access to only
their document library.

All of the default groups remain in tact.

I have assigned users to each of the groups, some users are in multiple groups.

The problem is that some users cannot see any of the document
libraries at all.  They authenticate and are taken to the BI Portal
site, but then can see no data beyond that.  (They can see the web
parts, but no data linked to those parts).

Other users, who are set up identically to the first users, can see
exactly what they are supposed to see.

I have had users reboot, clear cache/cookies/history, and there is no
change in behavior.  I have even put some users into the
administrators group to see if that would work, but still no change. 
It's as if SharePoint is ignoring the site groups for some users, and
not for others.

Anyone have any ideas on how to clear this up? 

Thanks in advance!